1. Field of the Invention
The present invention relates generally to containers for packaged explosives, and more particularly to a container useful for holding explosive boosters employed in downhole blasting.
2. Description of the Prior Art
In blasting, and particularly in downhole blasting, relatively insensitive explosives or blasting agents such as ammonium nitrate-fuel oil compositions, are detonated by booster or primer charges containing more sensitive explosive material which can be detonated by ordinary blasting caps and/or detonating cords. Heretofore, most such booster charges have comprised cast high explosives, such as trinitrotoluene (TNT) or pentaerythritol tetranitrate (PETN), melted and poured into a vessel. Such a vessel can be lowered to the bottom of a bore hole, and the blasting agent layered on top of it.
In many cases, it is desirable to place two or more charges of blasting agent at various elevations in the bore hole. To accomplish such layering (commonly referred to as "decking") it is necessary to separate the layers of blasting agent by intermediate layers of earth, rock, sand or other inert material. Such separation allows a short delay between the detonation of the various layers of explosives, which delay is desirable to achieve certain blasting patterns.
When layering or decking explosives, it is necessary that each separate charge of blasting agent have its own booster or primer charge for detonation. Then, by providing means for detonating the booster charges with a predetermined time delay between successive charges, the desired blasting sequence can be achieved.
Such isolated detonation can most easily be achieved by providing separate electric blasting caps for each level of blasting agent. An electric blasting cap is inserted into the booster, and the booster then lowered into the bore hole by means of the connecting electric cable. Separate blasting caps and cables are provided for each booster, and each booster in turn can be electrically detonated with any time delay desired. While electric blasting caps have the advantage of precise timing, the use of multiple cables complicates the detonation procedure. Moreover, electric blasting caps can be prematurely detonated by thunderstorms, stray electric current, static electricity, and RF energy. For these reasons, many users prefer non-electric detonation techniques.
Virtually all nonelectric detonation of booster charges is achieved using detonating cords which include an explosive core material, typically PETN, wherein the cord is initiated at the surface of the bore hole and rapidly propagates down into the hole. In the case of a single explosive charge in the hole, the detonating cord can be used to directly detonate the charge simply by inserting the cord into the primer charge. In this way, as soon as the detonation reaches the primer, the primer is detonated.
When multiple layers of explosive charges are to be detonated with time delays between each detonation, it is no longer possible to employ the detonating cord directly. Instead, a "transfer tube" is used to detonate a blasting cap which is inserted into the booster explosive. The transfer tube is typically tied at one end to the detonating cord. As the detonation propagates through the detonating cord and past the transfer tube, the transfer tube and the blasting cap are ignited. The blasting cap includes a delay element so that detonation of the primer charge does not occur for a preselected period.
Heretofore, a number of specialized booster containers have been developed which are capable of operation with delay-type transfer tubes. See, for example, the U.S. Pat. No. 4,178,852 to Smith et al. The explosive device described therein is currently sold by Atlas Powder Company, a subsidiary of Tyler Corporation, Dallas, Tex. 75251. The commercial unit is described in Data Sheet 701 of the Atlas Powder Company. The booster container of Smith et al. is intended to be detonated only by the particular delaying detonator described in the patent. While conventional detonating cord is used to suspend and ignite the booster, the transfer tube must have the particular configuration so that one end will lie adjacent to the detonating cord while the other end will be inserted in the detonator well formed into the container. While this container is functional, it does not allow initiation by electric blasting cap or direct initiation by the detonating cord.
Other specialized booster containers are described in U.S. Pat. Nos. 4,347,789; 4,334,476; 4,282,812; 4,023,494; 3,064,573; and 2,920,523. None of the booster containers described in these patents is useful for all the detonation techniques just described. For example, the downhole delay assembly described in U.S. Pat. No. 4,347,789 would not be useful for direct initiation by a detonating cord.